Resource recovery method using multi-stage submerged membrane distillation water treatment apparatus

ABSTRACT

A resource recovery method includes: feeding raw water to a first-stage raw water tank; supplying high-temperature vapor to a first-stage heat exchanger; performing heat exchange between the supplied high-temperature vapor and the raw water in the first-stage raw water tank, changing a portion of the water into vapor and supplying the changed vapor to a subsequent-stage heat exchanger; repeatedly performing the performing step for each of the raw water tanks sequentially in the order from a second state to a n-th stage; being feed to a crystallizer from the n-th stage raw water tank; detecting a turbidity of the raw water fed to the crystallizer from the n-th-stage raw water tank; and extracting crystals of valuable resources contained in the raw water fed to the crystallizer from the n-th-stage raw water tank when the turbidity of the raw water becomes a predetermined value.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional Application of a U.S. patentapplication Ser. No. 15/488,817 filed on Apr. 17, 2017, which claimspriority to Korean Patent Application Nos. 10-2016-0053058 filed on Apr.29, 2016, 10-2016-0078113 filed on Jun. 22, 2016, and 10-2017-0036694filed on Mar. 23, 2017, which are all hereby incorporated by referencein their entirety.

BACKGROUND

The present invention relates to a resource recovery method. Moreparticularly, the present invention relates to a resource recoverymethod using a multi-stage submerged membrane distillation watertreatment apparatus including a series of raw water tanks arranged inmultiple stages, each tank storing raw water and provided with amembrane distillation (MD) module and a heat exchanger submerged in theraw water, the apparatus being configured such that vapor dischargedfrom a membrane distillation module at a certain stage is used for heatexchange by a heat exchanger at the following stage, whereby theapparatus dramatically reduces consumption of energy required forheating of the raw water and recovers resources contained in the rawwater.

According to a 2009 World Economic Forum Water Resource Initiativereport, the world water demand increased three-fold from 1950 to 1990,which was an increase rate greatly higher than an increase rate of theworld population. It was also noted that water demand is expected toincrease two-fold within the next 35 years. In addition, the currentactive production of alternative energy resources such as hydrogen gasand bioethanol, will further increase water demand.

To address water shortages, recycling and reuse of sewage or waste waterhave recently been actively studied. However, in fact, a water reuserate is currently actually very low, or usage of recycled water is verylimited due to concerns about the quality of the recycle water. Forexample, recycled water is largely used as cleaning water in sewage andwaste water treatment facilities, or as cooling water or diluting water.

To solve this problem, various technologies are now being used in watertreatment or reuse sites. For example, membrane distillation (MD), whichis a water treatment technology of separating water in the form of purevapor from raw water using a hydrophobic porous membrane, is used.

In a membrane distillation process, raw water comes into contact withthe surface of a membrane but cannot permeate into the pores of themembrane due to high surface tension attributable to the highlyhydrophobic surface of the membrane, and only vapor passes through thepores of the membrane and is collected as fresh water.

The cause of mass transfer in a membrane distillation process is due toa temperature difference between high-temperature raw water andlow-temperature filtrate divided by a separation membrane. A vaporpressure difference triggered by the temperature difference is thedriving force of causing the vapor, changed from liquid phase water, tomove from the raw water side to the filtrate side.

Membrane distillation methods are classified into four categoriesaccording to technologies applied to the filtrate side to generate avapor pressure gradient serving as the driving force: direct contactmembrane distillation (DCMD); air gap membrane distillation (AGMD);sweep gas membrane distillation (SGMD); and vacuum membrane distillation(VMD).

Membrane distillation methods treat raw water based on phase change. Themethods have many advantages: approximately 100% of removal rate ofnonvolatile contaminants; lower operation pressures compared to reverseosmosis; and simple pretreatment equipment and facilities due to itshigh resistance to membrane contamination.

However, despite of these advantages, the methods also have adisadvantage that they require high energy consumption to heat raw waterto a predetermined temperature (typically 60 to 80° C.) to generate avapor pressure difference serving as the driving force of mass transfer.In addition, energy is further consumed to condense the vapor havingpassed through a membrane distillation module.

In addition, conventional membrane distillation methods have difficultyin removing contaminants from sewage or waste water in which highlyconcentrated salts (i.e. valuable resources) are contained, and are thuspoor at producing quality fresh water. Furthermore, conventionalmembrane distillation methods cannot recover or recycle valuableresources contained in high concentration, and thus the valuableresources are chemically treated and collected as sludge. As a result, alarge amount of chemicals is necessarily used and correspondingly alarge amount of sludge is generated, which results in an increase insludge treatment costs.

For these reasons, development of a technology of improving energyefficiency and efficiently recovering valuable resources contained inraw water in a membrane distillation process is required.

The foregoing is intended merely to aid in the understanding of thebackground of the present invention, and is not intended to mean thatthe present invention falls within the purview of the related art thatis already known to those skilled in the art.

DOCUMENTS OF RELATED ART

(Patent Document 1) Korean Patent Application Publication No.2014-0101589

(Patent Document 2) Korean Patent No. 1556915

(Patent Document 3) Korean Patent No. 1605535

SUMMARY

Accordingly, the present invention has been made in view of the problemsoccurring in the related arts, and an object of the present invention isto provide a resource recovery method using a multi-stage submergedmembrane distillation water treatment apparatus including a series ofraw water tanks arranged in multiple stages, the method removingcontaminants and high concentrations of salts contained in raw waterthrough a multi-stage membrane distillation water treatment process,thereby producing quality fresh water and recovering valuable resourcescontained in raw water while circulating raw water.

In order to accomplish the objects of the present invention, accordingto one aspect, there is provided a multi-stage submerged membranedistillation water treatment apparatus including: a plurality of rawwater tanks arranged in multiple stages ranging from a first stage to ann-th stage and storing raw water, the raw water sequentially flowingfrom the first stage to the n-th stage; a plurality of membranedistillation (MD) modules arranged in multiple stages and submerged inthe raw water in the respective raw water tanks, the MD modulesdischarging a portion of the raw water as vapor; a plurality of heatexchangers arranged in multiple stages and submerged in the raw water inthe respective raw water tanks, each heat exchanger performing heatexchange using the vapor supplied from a previous-stage MD module of theMD modules, thereby maintaining the raw water in the corresponding rawwater tank at a predetermined temperature; a vapor generator generatinghigh-temperature vapor and supplying the high-temperature vapor to thefirst-stage heat exchanger; a condenser condensing the vapor suppliedfrom the n-th-stage MD module through heat exchange, and dischargingfiltrate resulting from the condensation; and a raw water feeder feedinglow-temperature raw water to the first-stage raw water tank via thecondenser such that the row-temperature raw water is used for heatexchange in the condenser before being fed to the first-stage raw watertank.

The vapor generator may supply vapor having a temperature higher thanthat of the raw water in the first-stage raw water tank to thefirst-stage heat exchanger, and vapor that is discharged by thefirst-stage heat exchanger may be returned to the vapor generator.

The multi-stage submerged membrane distillation water treatmentapparatus may further include a filtrate tank storing vapor dischargedby the heat exchangers at the second to n-th stages and vapor dischargedby the condenser.

The multi-stage submerged membrane distillation water treatmentapparatus may further include: a temperature detector detecting atemperature of the raw water in the first-stage raw water tank; and avapor temperature controller controlling a temperature of the vaporgenerated by the vapor generator in accordance with the temperature ofthe raw water detected by the temperature detector.

The multi-stage submerged membrane distillation water treatmentapparatus may further include a plurality of aerators arranged inmultiple stages and submerged in the respective raw water tanks, theaerators performing aeration continuously or periodically.

According to another aspect, there is provided a multi-stage submergedmembrane distillation water treatment apparatus including: a pluralityof raw water tanks arranged in multiple stages ranging from a firststage to an n-th stage to store raw water, and configured such that theraw water sequentially flows from the first stage (foremost stage) tothe n-th stage (last stage); a plurality of membrane distillation (MD)modules arranged in multiple stages and submerged in the respective rawwater tanks, the MD modules discharging a portion of raw water as vapor;a plurality of heat exchangers arranged in multiple stages and submergedin the respective raw water tanks, each heat exchanger performing heatexchange using the vapor supplied from a previous-stage MD module of theMD modules, thereby enabling the raw water in the corresponding rawwater tank to be maintained at a predetermined temperature; a vaporgenerator generating high-temperature vapor and supplying the generatedhigh-temperature vapor to the heat exchanger in the first-stage rawwater tank; a crystallizer receiving raw water discharged from then-th-stage raw water tank and extracting crystals of valuable resourcescontained in the raw water; a turbidimeter detecting a turbidity of theraw water supplied to the crystallizer from the n-th-stage raw watertank; and a controller controlling operation of the crystallizer suchthat crystals of valuable resources in the raw water are collected whena turbidity detected by the turbidimeter is equal to a predeterminedreference value.

The crystallizer may selectively feed the raw water from which thecrystals are removed to at least one raw water tank among the raw watertanks at the first to n-th stages.

The multi-stage submerged membrane distillation water treatmentapparatus may further include: a condenser condensing vapor suppliedfrom the MD module in the n-th-stage raw water tank through heatexchange and discharging resultant condensate outside the condenser; anda raw water feeder feeding low-temperature raw water to the first-stageraw water tank via the condenser such that the low-temperature raw waterundergoes heat exchange in the condenser before being fed to thefirst-stage raw water tank.

The vapor generator may supply vapor having a temperature higher thanthat of the raw water in the first-stage raw water tank to thefirst-stage heat exchanger, and vapor that is used for heat exchange bythe first-stage heat exchanger and then discharged by the first-stageheat exchanger may be returned to the vapor generator.

The multi-stage submerged membrane distillation water treatmentapparatus may further include a filtrate tank storing vapor dischargedfrom the heat exchangers at the second to n-th stages after undergoingheat exchange, and vapor discharged from the condenser.

The multi-stage submerged membrane distillation water treatmentapparatus may further include: a temperature detector detecting atemperature of the raw water in the first-stage raw water tank; and avapor temperature controller controlling a temperature of the vaporgenerated by the vapor generator on the basis of the temperature of theraw water detected by the temperature detector.

The crystallizer may include: a housing receiving and storing raw waterpassing through the n-th-stage MD module; a filter installed in thehousing and filtering out crystals of predetermined valuable resourcescontained in the raw water; a ultrasonic generator generating apredetermined frequency of ultrasonic waves; and at least one ultrasonicvibrator attached to a portion of the housing to vibrate the housingusing the ultrasonic waves generated by the ultrasonic generator.

The controller may operate the ultrasonic generator when the turbidityof raw water detected by the turbidimeter becomes the predeterminedreference value, thereby vibrating the housing, transferring thevibration of the housing to the filter, and consequently separatingcrystals of valuable resources attached to the filter from the filter.

The crystallizer may further include a particle analyzer analyzing aparticle size of the valuable resources discharged outside the housing,in which the controller transmits a control signal for adjusting afrequency of ultrasonic waves generated by the ultrasonic generator onthe basis of the particle size of the valuable resources analyzed by theparticle analyzer, to the ultrasonic generator.

The crystallizer may further include a pressure gauge detecting aninternal pressure of the housing, wherein the controller operates thecrystallizer to separate and remove the crystals of valuable resourcesfrom the filter when the internal pressure of the housing reaches apredetermined critical pressure.

According to a further aspect, there is provided a resource recoverymethod using a multi-stage submerged membrane distillation watertreatment apparatus including: a plurality of membrane distillation (MD)modules arranged in multiple stages and submerged in raw water in aplurality of raw water tanks arranged in multiple stages ranging from afirst stage to an n-th stage, the MD modules discharging a portion ofthe raw water as vapor; and a plurality of heat exchangers arranged inmultiple stages, each heat exchanger performing heat exchange using thevapor supplied from a previous-stage MD module of the MD modules,thereby maintaining the raw water in each raw water tank at apredetermined temperature, the method comprising: a first step at whicha raw water feeder feeds raw water to the first-stage raw water tank; asecond step at which a vapor generator supplies high-temperature vaporto the first-stage heat exchanger; a third step at which the first stageheat exchanger performs heat exchange between the suppliedhigh-temperature vapor and the raw water in the first-stage raw watertank, and the first-stage MD module changes a portion of the water intovapor and supplying the changed vapor to a subsequent-stage heatexchanger (second-stage heat exchanger); a fourth step of repeatedlyperforming the third step for each of the raw water tanks sequentiallyin the order from the second state to the n-th stage; a fifth step atwhich the raw water is feed to a crystallizer from the n-th stage rawwater tank; a sixth step at which a turbidimeter detects a turbidity ofthe raw water fed to the crystallizer from the n-th-stage raw watertank; a seventh step at which a controller transits an operation signalto the crystallizer when the turbidity of the raw water detected by theturbidimeter becomes a predetermined reference value; and an eighth stepat which the crystallizer operated by the operation signal extractscrystals of valuable resources contained in the raw water fed to thecrystallizer from the n-th-stage raw water tank.

The eighth step may include: a first sub-step at which a ultrasonicvibrator generates a predetermined frequency of ultrasonic waves inresponse to the operation signal transmitted from the controller; asecond sub-step at which a ultrasonic vibrator attached to a portion ofa housing storing raw water supplied from the n-th-stage raw water tank,vibrates the housing using the ultrasonic waves generated by theultrasonic generator; and a third sub-step at which a filter installedin the housing is vibrated by vibration of the housing such thatcrystals of valuable resources, attached to the filter, are separatedfrom the filter.

After the third sub-step is performed, the following steps are furtherperformed: analyzing particle sizes of the crystals of valuableresources filtered out by the filter using a particle analyzer; andtransmitting a control signal for adjusting the frequency of ultrasonicwaves generated by the ultrasonic generator in accordance with theparticle sizes of the crystals of valuable resources analyzed by theparticle analyzer, to the ultrasonic generator.

After the first step, the following steps may be further performed:detecting a temperature of the raw water in the first-stage raw watertank, using a temperature detector; and controlling a temperature ofvapor generated by a vapor generator on the basis of the detectedtemperature of the raw water.

After the sixth step, the following steps may be further performed:detecting a conductivity of raw water fed to the crystallizer from then-th-stage raw water tank, using a conductivity meter; and transmittingan operation signal to the crystallizer by the controller when thedetected turbidity and the detected conductivity reach respectivereference values.

According to the present invention, the MD modules are submerged in therespective raw water tanks, connected in series, and arranged inmultiple stages. Therefore, latent energy of vapor discharged from aprevious-stage MD module is used to heat raw water at the followingstage. In addition, the vapor generated from the last-stage raw watertank is used to heat raw water to be introduced to the first-stage rawwater tank. Therefore, the apparatus of the present invention candramatically increase heat efficiency compared with conventionalmembrane distillation apparatuses.

In addition, according to the present invention, since the raw water ineach raw water tank is maintained at a constant temperature,high-temperature vapor can be produced. In addition, since latent energyof the high-temperature vapor is recycled, energy consumption can bedramatically reduced.

In addition, according to the present invention, since the membranedistillation water treatment apparatus is operated by only electricenergy, the apparatus has a simple structure and can be easilymaintained.

In addition, according to the present invention, since the aerators aresubmerged in the respective raw water tanks arranged in multiple stagesand continuously or periodically operated, contamination of the membranedistillation modules can be reduced.

In addition, according to the present invention, since valuableresources, conventionally regarded as waste, contained in raw water canbe recovered and recycled, it is possible to secure resources.Furthermore, it is possible to reduce usage of chemicals for recovery ofvaluable resources and reduce generation of sludge.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description when taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a block diagram of a multi-stage submerged membranedistillation water treatment apparatus according to one embodiment ofthe present invention;

FIG. 2 is a flowchart illustrating a multi-stage submerged membranedistillation water treatment method according to one embodiment of thepresent invention;

FIG. 3 is a flowchart illustrating a resource recovery method using themulti-stage submerged membrane distillation water treatment apparatusaccording to the embodiment of the present invention;

FIG. 4 is a flowchart illustrating the operation of a crystallizeraccording to one embodiment of the present invention; and

FIG. 5 is a block diagram of an experiment for estimating a totaldissolved solids (TDS) level and a recovery rate for each-stage rawwater tank.

DETAILED DESCRIPTION

Hereinafter, exemplary embodiments of the present invention will bedescribed with reference to the accompanying drawings. The samereference numerals throughout the drawings denote elements having thesame or similar function. In the following description of embodiments ofthe present invention, detailed descriptions of known functions andcomponents incorporated herein will be omitted when it may make thesubject matter of the present invention unclear.

It will be understood that, although the terms first, second, A, B, (a),(b) etc. may be used herein to describe various elements, these elementsshould not be limited by these terms. These terms are only used todistinguish one element from another. For example, a first element couldbe termed a second element, and, similarly, a second element could betermed a first element, without departing from the scope of exampleembodiments of the present invention. In addition, it will be understoodthat when an element is referred to as being “connected” or “coupled” toanother element, it can be directly connected or coupled to the otherelement or intervening elements may be present. In contrast, when anelement is referred to as being “directly connected” or “directlycoupled” to another element, there are no intervening elements present.Other words used to describe the relationship between elements should beinterpreted in a like fashion (e.g. “between” versus “directly between”,“adjacent” versus “directly adjacent”, etc.).

FIG. 1 is a block diagram of a multi-stage submerged membranedistillation water treatment apparatus according to one embodiment ofthe present invention.

With reference to FIG. 1, according to one embodiment of the presentinvention, a multi-stage submerged membrane distillation water treatmentapparatus 100 includes a plurality of raw water tanks 101 arranged inmultiple stages, a plurality of membrane distillation modules 102arranged in multiple stages to correspond to the raw water tanks 101, aplurality of heat exchangers 103 arranged in multiple stages tocorrespond to the raw water tanks 101, a vapor generator 104, acondenser 105, a raw water feeder 106, a turbidimeter 111, acrystallizer 113, and a controller 115.

The raw water tanks 101 are arranged in series and in multiple stagesranging from a first stage to an n-th stage. The raw water tanks 101store raw water. Raw water can be fed in various ways to the raw watertanks 101. For example, the raw water feeder 106 may feed raw water toall of the raw water tanks 101. Alternatively, the raw water feeder 106may feed raw water only to the raw water tank 101 a at the first stage(foremost stage), which is called a first-stage raw water tank 101 a,and the remaining raw water tanks 101 b to 101 n (i.e. from thesecond-stage raw water tank 101 b to the n-th-stage (last stage) rawwater tank 101 n) receive raw water respectively from the directlyprevious stage raw water tanks through raw water pipelines 114.

To this end, every two raw water tanks 101 adjacent to each other isconnected to each other via the raw water pipeline 114, so the raw watercan be sequentially fed from the first-stage raw water tank 101 a to then-th-stage raw water tank 101 n. The raw water also can be fed to thecrystallizer 113 from the n-th-stage (i.e. last stage) raw water tank101 n through the raw water pipeline 114, and the raw water dischargedfrom the crystallizer 113 may be selectively returned to the raw watertanks 101 a to 101 n.

The raw water can be drained to the outside from the n-th-stage (i.e.last stage) raw water tank 101 n at predetermined timing. The reason isas follows: during circulation of the raw water from the first-stage rawwater tank to the n-th-stage row water tank, a portion of the raw wateris changed into vapor for each raw water tank, and thus foreign matteraccumulates in the raw water tanks or the concentration of the foreignmatter in the raw water tanks gradually increases. For this reason, theraw water is discharged outside to address this problem.

The membrane distillation (MD) modules 102 are also arranged in multiplestages to correspond to the raw water tanks. Specifically, the MDmodules 102 are submerged in the raw water in the respective raw watertanks 101. A portion of the raw water supplied to an MD module 102 at acertain stage is changed into vapor and the vapor is supplied to a heatexchanger 103 at the subsequent stage (following stage). This processcorrespondingly applies to all the MD modules 102 and all the heatexchangers 103.

For example, the vapor generated by the MD module 102 a at the firststage (foremost stage) is supplied to the heat exchanger 103 b at thesecond stage subsequent to the first stage, and the vapor generated bythe MD module 102 b at the second stage is supplied to the heatexchanger 103 c at the third stage subsequent to the second stage. Inthis way, the vapor generated by the MD module 102 at an i-th stage issupplied to the heat exchanger 103 at an i+1-th stage subsequent to thei-th stage.

The vapor discharged from the MD module 102 n at the n-th stage (i.e.last stage) is supplied to the condenser 105 and condensed thereby.Since the next stage of the last stage (n-th stage) is not provided witha heat exchanger, the MD module 102 n at the last stage discharges thegenerated vapor to the condenser 105, and the vapor is condensed by thecondenser 105. The condensed vapor becomes filtrate and the filtrate issupplied to a filtrate tank 107.

Each MD module 102 is divided into a raw water region and a vapor regionby a membrane. When raw water comes into contact with one surface of themembrane, since the surface of the membrane is highly hydrophobic andhas high surface tension, the raw water cannot permeate into pores ofthe membrane. That is, only vapor can pass through the membrane andcollects in the vapor region. The vapor becomes fresh water later.

In the MD module 102, there is a temperature difference between the rawwater having a relatively low temperature and the vapor having a hightemperature divided by the membrane. Due to the temperature difference,the raw water of liquid phase changes into vapor and moves from the rawwater side to the filtrate (fresh water) side.

The heat exchangers 103 are also arranged in multiple stages tocorrespond to the raw water tanks and are submerged in the respectiveraw water tanks 101. The heat exchangers 103 receive the vapor from therespective previous-stage MD modules 102. At this point, the raw waterin each raw water tank 101 is heated through heat exchange with thevapor supplied in this way. Therefore, the raw water in each raw watertank 101 can be maintained at a predetermined temperature.

In this way, according to the present embodiment, the vapor dischargedfrom the MD modules 102 at certain stages is supplied to the heatexchangers 103 at the respective subsequent stages. Therefore,additional heaters for supplying thermal energy needed for heat exchangeperformed by the heat exchangers 103 at the following stages are notrequired. Therefore, energy consumption can be reduced. The supply ofthe vapor from a certain stage to the subsequent stage is sequentiallycarried out from the first stage (foremost stage) to the n-th stage(last stage).

The vapor generator 104 generates high-temperature vapor and suppliesthe high-temperature vapor to the heat exchanger 103 a at the firststage (foremost stage). That is, since there is no previous-stage MDmodule to supply vapor to the first-stage heat exchanger 103 a, thevapor generator 104 is used to supply high-temperature vapor to thefirst-stage heat exchanger 103 a.

The high-temperature vapor is heat-exchanged with the raw water by thefirst-stage heat exchanger 103 a. The raw water needs to be maintainedat a predetermined high temperature in order for the raw water to beeasily changed into vapor by the MD module. However, the raw waterdirectly fed from the raw water feeder 106 is relatively cold. For thisreason, the temperature of the raw water fed from the raw water feeder16 to the first-stage raw water tank needs to be increased. That is, theraw water needs to be heat-exchanged with the high-temperature vapor. Toenable this heat exchange, the vapor generator 104 functions to supplythe high-temperature vapor to the first-stage heat exchanger 103 a.

In this case, the vapor generator 104 preferably supplieshigh-temperature vapor having a temperature higher than that of the rawwater in the first-stage raw water tank 101 a, to the first-stage heatexchanger 103 a. This is because the temperature of the raw water can beincreased through heat exchange between the vapor and the raw water in acondition in which the temperature of the vapor is higher than thetemperature of the raw water.

Next, after the heat exchange is performed by the first-stage heatexchanger 103 a, the vapor is returned to the vapor generator 104. Thehigh-temperature vapor supplied to the first-stage heat exchanger 103 aby the vapor generator 104 is discharged as low-temperature vapor afterit undergoes the heat exchange with the raw water. This low-temperaturevapor is returned to the vapor generator 104 and then heated by thevapor generator 104 to a predetermined temperature. The heated vapor issent again to the first-stage heat exchanger 103 a. In this way, thevapor supplied to the first-stage heat exchanger 103 a is managed to bealways maintained at a high temperature.

The condenser 105 condenses the vapor supplied by the n-th-stage (i.e.last stage) MD module 102 n through heat exchange and discharges thecondensed vapor as filtrate to the filtrate tank 107. For this heatexchange, the condenser 105 is supplied with low-temperature raw waterby the raw water feeder 106. Thus, the vapor supplied by the n-th-stageMD module 102 n to the condenser 105 is heat-exchanged with thelow-temperature raw water supplied by the raw water feeder 106. Throughthis heat exchange, the vapor is condensed and then discharged asfiltrate to the filtrate tank 107.

The raw water feeder 106 feeds the low-temperature raw water to thefirst-stage (i.e. foremost stage) raw water tank 101 a via the condenser105. That is, when the low-temperature raw water is supplied thefirst-stage raw water tank 101 a, since the low-temperature water issupplied via the condenser 105, condensation of vapor can be performedby the condenser 105.

According to the embodiment described above, the raw water feeder 106feeds raw water to only the first-stage raw water tank 101 a. However,according to another embodiment, the raw water feeder 106 can feed rawwater to all the raw water tanks 101 a to 101 n. In that case, the rawwater feeder 106 may feed raw water to the raw water tanks 101 using apump (not shown).

As described above, according to the embodiments of the presentinvention, the MD modules 102 and the heat exchangers 103, both arrangedin multiple stages, are submerged in raw water in the raw water tanks101 arranged in multiple stages, and raw water and vapor aresequentially and repeatedly moved from one stage to another so thatfiltrate can be continuously produced.

Specifically, the vapor discharged from the MD modules 102 arranged inmultiple stages is not used to produce fresh water but used for heatexchange by the heat exchangers 103 at the respective subsequent stages,and the first-stage heat exchanger performs heat exchange with thehigh-temperature vapor supplied by the vapor generator 104. Due to thisoperation process, additional heaters to supply thermal energy neededfor heat exchange of the heat exchangers 103 are not necessary.Therefore, energy consumption can be dramatically reduced.

In the present invention, the vapor used for heat exchange by thefirst-stage heat exchanger 103 a is returned to the vapor generator 104,but the vapor used for heat exchange by the remaining heat exchangers(i.e. from the second-stage heat exchanger 102 b to the n-th-stage heatexchanger 102 n) is supplied to the filtrate tank 107.

That is, the filtrate tank 107 receives and stores the vapor in thisway. The filtrate tank 107 also receives and stores the vapor dischargedfrom the n-th-stage MD module 102 n. The vapor discharged from then-th-stage MD module 102 n is supplied to the filtrate tank 107 via thecondenser 105. When the vapor passes through the condenser 105, thevapor undergoes heat exchange with the low-temperature raw water fed bythe raw water feeder 106, thereby being condensed. Afterwards, thecondensed vapor is sent to the filtrate tank 107.

The crystallizer 113 receives the raw water discharged from then-th-stage raw water tank 101 n via the raw water pipeline 114 andextracts and separates crystals of specific substances, i.e. valuableresources contained in the raw water. The separated crystals aredischarged outside the crystallizer and the raw water from which thecrystals of valuable resources are removed is selectively returned tothe raw water tanks 101 a to 101 n.

As shown in the drawings, according to one embodiment of the presentinvention, the crystallizer 113 includes a housing 1131, a filter 1132,a ultrasonic generator 1133, and a ultrasonic vibrator 1134.

The housing 1131 has an internal space with a predetermined size tostore the raw water discharged from the n-th-stage raw water tank 101 n.

The filter 1132 is present in plural number and is installed in thehousing 1131. The filter 1132 separates the crystals of predeterminedvaluable resources contained in the raw water by filtering out thecrystals of the predetermined valuable resources. The filter 1132includes a cartridge filter, for example. Since, as described above, aportion of the raw water is vaporized while it circulates through thewater treatment apparatus, the amount of the raw water decreases withtime. Therefore, the concentration of valuable resources in the rawwater increases with time and finally the valuable resources aresupersaturated in the raw water, thereby being precipitated as crystals.The filter 1132 filters out the crystals, thereby separating thecrystals from the raw water.

That is, the crystals of valuable resources are filtered out by thefilter 1132, and only the raw water passes through the filter 1132. Thefiltered raw water is selectively returned to at least one raw watertank of the raw water tanks 101 a to 101 n arranged in multiple stages.Here, which raw water tanks are supplied with the filtered raw water isdetermined preferably in accordance with a concentration of the rawwater. For example, it is preferable that the raw water tanks, in whichraw water has a concentration equal or similar to that of the raw waterfiltered by the filter 1132, are supplied with the raw water filtered bythe filter 1132. For example, the concentration of the raw water meansthe level (mg/L) of total dissolved solids (TDS) in the raw water. Theraw water discharged from the crystallizer 113 is preferably fed to theraw water tanks storing raw water having a TDS level similar to that ofthe raw water discharged from the crystallizer 113.

Meanwhile, in the present embodiment, during the process in which thecrystals of valuable resources are filtered out by the filter 1132, thecrystals are likely to stick to the surface of the filter 1132. For thisreason, according to the present invention, the filter 1132 is vibratedso that the crystals of the valuable resources can be easily detachedfrom the surface of the filter 1132. For vibration of the filter 1132,according to the present invention, the ultrasonic generator 1133generates a predetermined frequency of ultrasonic waves and transmitsthe ultrasonic waves to the ultrasonic vibrator 1134 attached to aportion of the housing 1131. With the use of the ultrasonic wavesgenerated by the ultrasonic generator 1133, the ultrasonic vibrator 1134causes the housing 1131 to vibrate.

The vibration of the housing 1131 is transferred to the filter 1132 viathe raw water, so the filter 1132 is also vibrated. The crystals ofvaluable resources attached to the surface of the filter 1132 aredetached and removed from the filter 1132 due to the vibration of thefilter 1132. Subsequently, the raw water from which valuable resourcesare removed is selectively returned to the raw water tanks 101 a to 101n, and the removed valuable resources are discharged outside thecrystallizer.

According to another embodiment of the present invention, thecrystallizer 113 may further include a stirrer 1138 installed in thehousing 1131. The stirrer rotates the crystals of valuable resourcesremaining in the housing 1131, thereby increasing the size of thecrystals and consequently causing the crystals to be collected at thebottom of the housing 1131 by centrifugal force. For this reason, it ispreferable that the housing 1131 has a conical shape.

In addition, the housing 1131 is preferably made of stainless steel sothat the vibration of the housing 1131 can be more easily transferred tothe filter 1132.

In addition, according to a further embodiment of the present invention,the crystallizer 113 may further include a particle analyzer 1135 thatanalyzes the particle sizes of the crystals of valuable resourcesdischarged from the housing 1131. In this case, the controller 115transmits a control signal for adjusting the frequency of ultrasonicwaves generated by the ultrasonic generator 1133 in accordance with theparticle sizes of the crystals of valuable resources, analyzed by theparticle analyzer 1135, to the ultrasonic generator 1133.

This control is performed to generate a higher frequency of ultrasonicwaves for valuable resources with a smaller particle size, therebyincreasing a recovery rate of valuable resources. In addition, thecontroller 115 determines timing at which valuable resources arerecovered on the basis of the results of the particle size analysis ofthe particle analyzer 1135, and controls opening of a discharge valve1136 installed in a lower portion of the housing 1131, therebyrecovering valuable resources.

The turbidimeter 111 continuously detects the turbidity of raw waterthat is fed to the crystallizer 113 from the n-th-stage (i.e. laststage) raw water tank 101 n. The turbidity of raw water is used todetermine timing at which crystals of valuable resources are formed inthe crystallizer 113. That is, when crystals of valuable resources startto form, the turbidity of raw water becomes a predetermined value.

For example, when crystals of highly concentrated salts, i.e. valuableresources, start being formed in the raw water, the raw water becomesturbid. At this point, the turbidimeter 111 detects the predeterminedvalue. That is, detection of the predetermined value by the turbidimeter111 means that crystals of valuable resources are formed. Therefore, atthis point, the crystallizer 113 causes the filter 1132 to vibrate toprevent the crystals of valuable resources from sticking to the filter1132, thereby enabling the crystals of valuable resources to be easilyseparated and removed from the raw water.

The controller 115 determines whether the turbidity detected by theturbidimeter 111 is equal to the predetermined value, i.e. referencevalue, then determines that crystals of valuable resources are formedwhen the detected turbidity is equal to the reference value, andtransmits an operation signal to the ultrasonic generator 1133 of thecrystallizer 113. Next, the ultrasonic generator 1133 generatesultrasonic waves and transfers ultrasonic waves to the ultrasonicvibrator 1134, thereby causing vibration of the housing 1131. Thevibration of the housing 1131 is transferred to the filter 1132 via theraw water, so the filter 1132 is accordingly vibrated. In consequence,the crystals of valuable resources on the surface of the filter 1132 aredetached from the filter 1132.

According to a further embodiment, the controller 115 receives apressure value detected by a pressure gauge 1137 installed in thehousing 1131 and causes the filter 1132 of the crystallizer 113 to becleaned when the detected pressure value reaches a predeterminedcritical pressure value.

The cleaning of the filter 1132 is performed to detach the crystals ofvaluable resources from the surface of the filter 1132. The cleaning isperformed by operating the ultrasonic generator 1133 and causingvibration of the filter 1132.

In addition, according to a further embodiment of the present invention,the multi-stage submerged membrane distillation water treatmentapparatus 100 may further include a conductivity meter 112. Theconductivity meter 112 detects the conductivity of raw water fed to thecrystallizer 113 from the n-th-stage raw water tank 101 n. In this case,the controller 115 determines that the crystals of valuable resourcesare formed when the turbidity detected by the turbidimeter 111 and theconductivity detected by the conductivity meter 112 reach respectivepredetermined reference values, and controls the crystallizer 113 suchthat the crystals of valuable resources can be removed from thecrystallizer 113.

According to this embodiment, it is possible to more precisely determinetiming at which the crystals of valuable resources are formed, by usingthe conductivity of raw water. Therefore, it is possible to moreprecisely and accurately determine operation timing of the crystallizer113.

In addition, the multi-stage submerged membrane distillation watertreatment apparatus may further include a temperature detector 109 todetect a temperature of the raw water in the first-stage raw water tank101 a, and a vapor temperature controller 110 to control the operationof the vapor generator 104 on the basis of the temperature detected bythe temperature detector 109. The multi-stage submerged membranedistillation water treatment apparatus 100 may further include a vacuumpump 115 or a plurality of aerators 108.

The temperature detector 109 detects the temperature of the raw water inthe first-stage raw water tank 101 a in real time, and transmits thedetected temperature to the vapor temperature controller 110. The vaportemperature controller 110 controls the operation of the vapor generator104 in accordance with the detected temperature such that thetemperature of the raw water in the first-stage raw water tank 101 abecomes a predetermined reference temperature.

For example, when the temperature of the raw water in the first-stageraw water tank 101 a is lower than the reference temperature, the vaportemperature controller 110 controls the vapor generator 104 such thatrelatively high-temperature vapor is generated by the vapor generator104. Conversely, when the temperature of the raw water is higher thanthe reference temperature, the vapor temperature controller 110 controlsthe vapor generator 104 such that relatively low-temperature vapor isgenerated by the vapor generator 104. This control is to maintain theraw water in the first-stage raw water tank 101 a at the predeterminedreference temperature.

The vacuum pump 115 creates a vacuum pressure in the vapor region of theMD module 102. The MD modules 102 may include various types ofmembranes. For example, in the embodiments of the present invention, theMD module 102 preferably includes a membrane for vacuum membranedistillation but it is not limited thereto. That is, any known type ofmembrane can be used if the membrane can change raw water into vapor anddischarge the vapor.

For example, in order to obtain a vacuum membrane distillation (VMD)module, the vacuum pump 115 is used to create a vacuum pressure in thevapor regions of the MD modules 102 at the respective stages.

According to the embodiments, after the raw water is fed to the rawwater tanks 101 at the respective multiple stages, the pressure in thevapor regions of the MD modules 102 is maintained to be lower than vaporpressure by using the vacuum pump 115. The amount and temperature ofvapor generated by each MD module 102 are estimated according toEquations 1 to 4, and vapor pressure at each stage can be accordinglyadjusted. First, for each MD module 102, a wafer permeation flux Jw thatis a water production amount (m³) per unit area (m²) and unit time (hr),and vapor temperature (Tm) can be obtained using Equations 1 through 3.

J _(w) =A _(B)[P _(v)(T _(m) ,C _(m))−P ₀]  Equation 1

Wherein, Jw is a water permeation flux, AB is a water permeabilitycoefficient of an MD module, Pv is a vapor pressure, Tm is a temperatureof raw water and vapor on the surface of a membrane of an MD module, Cmis a salt concentration on the surface of a membrane of an MD module,and P0 is a vacuum pressure.

$\begin{matrix}{J_{w} = {k\; {\ln \left( \frac{C_{m}}{C_{b}} \right)}}} & {{Equation}\mspace{14mu} 2}\end{matrix}$

Wherein, k is a mass transfer coefficient, ln is a natural logarithm,and Cb is a salt concentration of raw water.

$\begin{matrix}{T_{m} = {\frac{J_{w}\Delta \; H_{v}}{h_{w}} - T_{b}}} & {{Equation}\mspace{14mu} 3}\end{matrix}$

Wherein, ΔHv is latent heat of vaporization, hw is a heat transfercoefficient, and Tb is a temperature of raw water.

Here, the vapor pressure of raw water can be calculated according toEquation 4. The production amount and temperature of vapor can beadjusted with adjustment of the vacuum pressure P0.

$\begin{matrix}{{P_{v}\left( {T_{m},C_{m}} \right)} = \frac{\exp \begin{Bmatrix}\begin{matrix}{\frac{{- 5.80} \times 10^{3}}{T_{m}} +} \\{1.39 - {4.86 \times 10^{- 2}T_{m}} + {4.18 \times}}\end{matrix} \\{{10^{- 5}T_{m}^{2}} + {{- 1.45} \times 10^{- 8}T_{m}^{2}} + {6.55{\log\left( T_{m)} \right.}}}\end{Bmatrix}}{1 + {0.57257\left( \frac{C_{m}}{1000 - C_{m}} \right)}}} & {{Equation}\mspace{14mu} 4}\end{matrix}$

The aerators 108 are submerged in the raw water in the respective rawwater tanks 101 and continuously or periodically operated. The aerationis performed to remove contaminants attached to the membranes of the MDmodules 102. For example, the aerators 108 remove the contaminantsattached to the membranes by generating air bubbles in the raw water.

FIG. 2 is a flowchart illustrating a multi-stage submerged membranedistillation water treatment method according to one embodiment of thepresent invention.

With reference to FIG. 2, the multi-stage submerged membranedistillation water treatment method according to one embodiment of thepresent invention is implemented in a multi-stage submerged membranedistillation water treatment apparatus having a structure in which rawwater is stored in a plurality of raw water tanks 101 a to 101 narranged in multiple stages ranging from a first stage to an n-th stage,a plurality of MD modules 102 a to 102 n is submerged in the respectiveraw water tanks 101 a to 101 n, and a plurality of heat exchangers 103 ato 103 n is submerged in the respective raw water tanks 101 a to 101 n.

A raw water feeder 106 feeds raw water to the first-stage raw water tank101 a (Step S101), and a vapor generator 104 supplies high-temperaturevapor to the first-stage heat exchanger 103 a (Step S103). Thefirst-stage heat exchanger 103 a performs heat exchange between thesupplied high-temperature vapor and low-temperature raw water (StepS105). Through this heat exchange, the raw water in the first-stage rawwater tank 101 a can be maintained at a predetermined high temperature.The vapor used for heat exchange by the first-stage heat exchanger 103 ais returned to the vapor generator 104.

Next, the first-stage (foremost-stage) MD module 102 a changes a portionof high-temperature raw water into vapor (Step S107), and supplies thevapor to the subsequent-stage (second-stage) heat exchanger 103 b (StepS109). The second-stage heat exchanger 103 b performs heat exchangebetween the supplied vapor and the raw water in the second-stage rawwater tank 101 b (Step S111). In this way, the temperature of the rawwater is increased through heat exchange.

Step S107 through Step S111 are repeatedly performed for the remainingraw water tanks in the order from the second-stage raw water tank to then-th-stage (i.e. last stage) raw water tank (Step S113). That is, an MDmodule 102 at a certain stage supplies high-temperature vapor to a heatexchanger 1043 at the subsequent stage, and the heat exchanger 103 atthe subsequent stage performs heat exchange between the supplied vaporand the raw water. This process is sequentially performed for each rawwater tank in the order from the second stage to the n-th stage, i.e.last stage.

After Step S107 through Step S111 are sequentially and repeatedlyperformed, that is, when the heat exchange by the n-th-stage (laststage) heat exchanger 103 n is finished (Step S115), the vapordischarged from the n-th-stage MD module 102 n is supplied to thecondenser 105 (Step S117) because there is no stage subsequent to then-th stage. The condenser 105 condenses the vapor through heat exchangebetween the vapor discharged from the n-th stage MD module 102 n andlow-temperature water fed by the raw water feeder 106 (Step S110), andsupplies the condensed vapor to a filtrate tank 107 (Step S121). Thatis, since there is no stage subsequent to the n-th stage (i.e. laststage), the vapor discharged from the n-th-stage MD module 102 n issupplied to the condenser 105, and the vapor is heat-exchanged with thelow-temperature raw water, fed by the raw water feeder 106, in thecondenser 105. Through this heat exchange, the vapor is condensed tobecome filtrate and the filtrate is supplied to the filtrate tank 107.

In addition, although not shown in the drawings, all of the raw watertanks 101 a to 101 n at the respective stages can drain the raw water tothe outside. That is, when foreign matter accumulates or is highlyconcentrated in the raw water while the raw water circulates through theraw water tanks, the raw water in the raw water tanks 101 a to 101 n isdischarged outside.

In addition, a plurality of aerators 108 may be submerged in therespective raw water tanks 101 and may be continuously or periodicallyoperated. Through this aeration, contaminants attached to the MD modules102 at the respective stages can be removed.

As described above, in the multi-stage submerged membrane distillationwater treatment apparatus 100 according to the embodiment of the presentinvention, raw water circulates through the raw water tanks 101 a to 101n arranged in multiple stages and sequentially undergoes membranedistillation in each raw water tank. Through the membrane distillation,vapor is generated from the raw water and is then changed into freshwater. Through this process, quality fresh water can be produced. Inaddition, timing at which crystals of valuable resources are formed inthe raw water is detected on the basis of a turbidity and a conductivityof the raw water, and the valuable resources are extracted using thecrystallizer.

FIG. 3 is a flowchart illustrating a resource recovery method using amulti-stage submerged membrane distillation water treatment apparatusaccording to one embodiment of the present invention.

With reference to FIG. 3, according to the resource recovery methodusing a multi-stage submerged membrane distillation water treatmentapparatus according to one embodiment of the present invention, the rawwater feeder 106 feeds low-temperature raw water to the first-stage(i.e. foremost stage) raw water tank 101 a (Step S201). Next, the vaporgenerator 104 supplies high-temperature vapor to the first-stage heatexchanger 103 a (Step S203). The first-stage heat exchanger 103 aperforms heat exchange between the high-temperature vapor and thelow-temperature raw water (Step S205). Through this heat exchange, theraw water in the first-stage raw water tank 101 a can be maintained at apredetermined high temperature. The vapor having undergone the heatexchange in the first-stage heat exchanger 103 a is returned to thevapor generator 104.

Next, the first-stage MD module 102 a changes a portion of thehigh-temperature raw water into vapor (Step S207) and supplies the vaporto the second-stage (i.e. subsequent stage) heat exchanger 103 b (StepS209). The second-stage heat exchanger 103 b performs heat exchangebetween the vapor supplied from the first-stage MD module 102 a and theraw water in the second-stage raw water tank 101 b (Step S211). Throughthis heat exchange, the temperature of the raw water in the second-stageraw water tank is increased.

S207 through S211 are sequentially and repeatedly performed for all ofthe remaining raw water tanks in the order from the second-stage rawwater tank to the n-th-stage (i.e. last stage) raw water tank (StepS213). That is, high-temperature vapor is supplied from an MD module 102at a certain stage to a heat exchangers 103 at the subsequent stage, andthe heat exchanger 103 at the subsequent stage perform heat exchangebetween the supplied vapor and the raw water. This process issequentially performed from the second stage to the n-th stage.

When Step S207 through S211 are continuously performed and finally theheat exchange of the n-th stage (i.e. last stage) heat exchanger 103 nis finished (Step S215), the raw water in the n-th-stage raw water tank101 n is supplied to the crystallizer 113 via the raw water pipeline 114(Step S217). At this point, the turbidimeter 111 detects the turbidityof the raw water supplied to the crystallizer 113 from the n-th-stageraw water tank 101 n (Step S219).

Next, the controller 115 compares the turbidity of the raw waterdetected by the turbidimeter 111 with a predetermined reference value(Step S221). When the detected turbidity is equal to the referencevalue, the controller 115 determines that crystals of valuable resourcesare formed in the raw water and transmits an operation signal to thecrystallizer 113 to recover the crystals of valuable resources generatedin the raw water (Step S223).

The crystallizer 113 is operated in response to the operation signal,thereby separating and discharging the crystals of valuable resourcescontained in the raw water (Step S225). This method determines timing atwhich valuable resources are concentrated and supersaturated to beprecipitated as crystals during circulation of the raw water, throughthe turbidity detection, and operates the crystallizer 113 to filter outand separate the valuable resources when the results of the turbiditydetection show the timing.

Next, the raw water from which valuable resources are removed isreturned to the raw water tanks 101 a to 101 n at the respective steps(Step S227), and the processes described above are repeated. Since aportion of the raw water is changed into vapor during circulation of theraw water and the amount of the raw water decreases with time, the rawwater in the raw water tanks is replenished by the raw water feeder 111at predetermined times.

In addition, although not shown in the drawings, according to anotherembodiment, the conductivity of the raw water supplied to thecrystallizer 113 from the n-th-stage raw water tank 101 n may bedetected by the conductivity meter 112 after the turbidity is detected.In this case, the controller 115 operates the crystallizer 113 when thedetected turbidity and the detected conductivity reach respectivereference values to recover crystals of valuable resources.

FIG. 4 is a flowchart illustrating the operation of the crystallizeraccording to one embodiment of the present invention.

With reference to FIG. 4, according to one embodiment of the presentinvention, the crystallizer determines whether it has received anoperation signal from the controller 115 (Step S301), and generates apredetermined frequency of ultrasonic waves by operating the ultrasonicgenerator 1133 when the operation signal is received (Step S303).

The ultrasonic waves are transferred to at least one ultrasonic vibrator1134 attached to a portion of the housing 1131 of the crystallizer 113(Step S305), so the housing 1131 is vibrated (Step S307).

The vibration of the housing 1131 is transferred to the filter 1132 inthe housing 1131 via the raw water, so the filter 1132 is also vibrated(Step S309). Therefore, the crystals of valuable resources attached tothe filter 1132 can be separated from the filter 1132 (Step S311). Atthis point, the raw water from which the crystals of valuable resourcesare removed is returned to the raw water tanks 101.

Although not illustrated in the drawings, after the crystals of valuableresources are separated from the filter 1132, the particle sizes of thecrystals of valuable resources are analyzed using the particle analyzer1135. In this case, the controller 115 transmits a control signal foradjusting the frequency of ultrasonic waves generated by the ultrasonicgenerator 1133 in accordance with the particle size analyzed by theparticle analyzer 1135, to the ultrasonic generator 1133.

In addition, the controller 115 determines timing at which the crystalsof valuable resources are recovered, in accordance with the analyzedparticle size, and recovers the valuable resources by opening andclosing the discharge valve 1136. In addition, the controller 115 mayoperate the crystallizer 113 to perform cleaning of the filter 1132 whena pressure value detected by the pressure gauge 1137, which isconfigured to detect the internal pressure of the housing 1131, reachesa predetermined critical pressure value.

As described above, in the multi-stage submerged membrane distillationwater treatment apparatus and method according to the present invention,vapor discharged by an MD module at a certain stage is supplied to aheat exchanger at the subsequent stage. That is, the vapor at a previousstage is used for heat exchange at the following stage. In addition, aportion of the vapor discharged from the MD module at the last stage iscompressed by a vapor compressor to be changed into high-temperaturevapor, and the high-temperature vapor is supplied to the heat exchangerat the first stage so as to be used for heat exchange at the firststage. In this way, since the vapor at previous stages is recycled andreused at the following stages, energy consumption can be dramaticallyreduced compared with conventional apparatuses. Furthermore, sinceadditional heat sources for heat exchange with the raw water are notrequired, the apparatus of the present invention has advantages of lowequipment cost, simple system, and easy maintenance.

In addition, in the resource recovery method using the multi-stagesubmerged membrane distillation water treatment apparatus according toone embodiment of the present invention, when crystals of valuableresources are formed during circulation of the raw water through themultistage raw water tanks, the crystals of valuable resources arecollected through filtering in the crystallizer. Therefore, valuableresources are not discharged but collected. That is, since valuableresources, contained in waste water or sewage and conventionallyregarded as waste, are recovered and recycled, resources can be secured.Furthermore, usage of chemicals used for recovery of valuable resourcescontained in raw water and production of sludge can be reduced.

FIG. 5 is a diagram illustrating an experiment for estimating a totaldissolved solids (TDS) level and a recovery rate for each-stage rawwater tank according to one embodiment of the present invention.

As described above, the raw water discharged from the crystallizer 113is returned to some selected raw water tanks of the first to n-th rawwater tanks. Preferably, the raw water discharged from the crystallizer113 is returned to the raw water tanks storing raw water in which aconcentration of raw water is equal to or similar to that of the rawwater discharged from the crystallizer 113. This process is performed toreuse the raw water discharged from the crystallizer 113 by mixing theraw water discharged from the crystallizer 113 with raw water having asimilar or equal concentration. This return process is continued untilthe concentration of raw water in each raw water tank reaches apredetermined reference value. When the concentration of the raw waterin each raw water tank reaches the predetermined reference value, theraw water is drained to the outside.

With reference to FIG. 5, raw water is sequentially supplied in theorder from the first-stage raw water tank 101 a to the n-th-stage rawwater tank 101 n through the raw water pipelines 114. In this case, aninitial TDS level of raw water fed to the first-stage raw water tank 101a is set to TDS(0). The TDS levels of the raw water in the raw watertanks 101 a to 101 n after a portion of the raw water is vaporized isrespectively set to TDS(1), TDS(2), TDS(3), . . . , and TDS(n). Whenrecovery rates for the raw water tanks 101 a to 101 n are respectivelyset to Rec(1), Rec(2), Rec(3), . . . , and Rec(n), the TDS levels forthe raw water tanks 101 a to 101 n are calculated according to Equation5.

TDS(n)=TDS(n−1)×(1/(1−Rec(n)/100))×(1−Rec(n)/100)+TDS(n−1)×(Rec(n)/100)  Equation5

Wherein, the recovery rate Rec at one stage is calculated as “(a waterproduction amount of a raw water tank/an amount of raw water fed the rawwater tank)×100”. That is, the recovery rate for a certain stage can beadjusted with change in the water production amount of the stage.Specifically, the production amount can be calculated by multiplying thewater permeation flux, calculated by Equations 1 through 4, by an areaof a membrane of an MD module submerged in raw water at the stage. Inthis calculation, since the amount of raw water fed to a raw water tankis a preset value, the recovery rate can be decided according to thewater production amount.

Table 1 shows TDS levels of raw water in 7 raw water tanks and totalrecovery rates obtained through an exemplary experiment of the presentinvention.

TABLE 1 Raw Recov- Concen- TDS TDS in water ery tration after raw Totaltank Inflow rate coeffi- concen- water recovery No. TDS (Rec) cienttration tank rate 1 35,000 20% 1.25 43,750 42,000 20.0% 2 42,000 20%1.25 52,500 54,400 36.0% 3 50,400 20% 1.25 63,000 60,480 48.8% 4 60,48020% 1.25 75,600 72,576 59.0% 5 72,576 20% 1.25 90,720 87,091 67.2% 687,091 20% 1.25 108,864 104,509 73.8% 7 104,509 20% 1.25 130,636 125,41179.0%

The TDS level of raw water in each raw water tank is estimated throughthis process, and compared with the TDS level of the raw waterdischarged from the crystallizer 113. Thus, the raw water dischargedfrom the crystallizer 113 is returned to the raw water tanks storing rawwater having a TDS level most similar to that of the raw waterdischarged from the crystallizer 113.

Although hereinabove a description has been made such that all elementsare combined in one embodiment or operated in combination, the presentinvention is not limited to such a configuration. That is, only someelements of all the elements can be selectively combined or can beoperated in combination within the scope of the objects of the presentinvention. It will be further understood that the terms “comprise”,“include”, “have”, etc. when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,components, and/or combinations of them but do not preclude the presenceor addition of one or more other features, integers, steps, operations,elements, components, and/or combinations thereof. Unless otherwisedefined, all terms including technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs. It will be further understoodthat terms, such as those defined in commonly used dictionaries, shouldbe interpreted as having a meaning that is consistent with their meaningin the context of the relevant art and the present disclosure, and willnot be interpreted in an idealized or overly formal sense unlessexpressly so defined herein.

Although a preferred embodiment of the present invention has beendescribed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the substantial nature of theinvention. Accordingly, exemplary embodiments disclosed herein have notbeen described for limiting the technical spirit of the presentinvention but be described only for illustrative purposes. Therefore,the technical spirit of the present invention will not be limited to theexemplary embodiments. The specific protection scope of the presentinvention should be construed as defined by the accompanying claims, andall technical spirits equivalent thereto should be construed to bewithin the protection scope of the present invention.

What is claimed is:
 1. A resource recovery method using a membranedistillation water treatment apparatus including: a plurality ofmembrane distillation (MD) modules arranged in multiple stages rangingfrom a first stage to an n-th stage and submerged in raw water inrespective raw water tanks arranged in multiple stages ranging from thefirst stage to the n-th stage, the MD modules discharging a portion ofthe raw water as vapor; and a plurality of heat exchangers arranged inmultiple stages ranging from the first stage to the n-th stage, eachheat exchanger performing heat exchange using the vapor supplied from aprevious-stage MD module of the MD modules, thereby maintaining the rawwater in each raw water tank at a predetermined temperature, the methodcomprising: a first step at which a raw water feeder feeds raw water tothe first-stage raw water tank; a second step at which a vapor generatorsupplies high-temperature vapor to the first-stage heat exchanger; athird step at which the first stage heat exchanger performs heatexchange between the supplied high-temperature vapor and the raw waterin the first-stage raw water tank, and the first-stage MD module changesa portion of the water into vapor and supplying the changed vapor to asubsequent-stage heat exchanger (second-stage heat exchanger); a fourthstep of repeatedly performing the third step for each of the raw watertanks sequentially in the order from the second state to the n-th stage;a fifth step at which the raw water is feed to a crystallizer from then-th stage raw water tank; a sixth step at which a turbidimeter detectsa turbidity of the raw water fed to the crystallizer from the n-th-stageraw water tank; a seventh step at which a controller transits anoperation signal to the crystallizer when the turbidity of the raw waterdetected by the turbidimeter becomes a predetermined reference value;and an eighth step at which the crystallizer operated by the operationsignal extracts crystals of valuable resources contained in the rawwater fed to the crystallizer from the n-th-stage raw water tank.
 2. Theresource recovery method according to 1, wherein the eighth stepcomprises: a first sub-step at which a ultrasonic vibrator generates apredetermined frequency of ultrasonic waves in response to the operationsignal transmitted from the controller; a second sub-step at which aultrasonic vibrator attached to a portion of a housing storing raw watersupplied from the n-th-stage raw water tank, vibrates the housing usingthe ultrasonic waves generated by the ultrasonic generator; and a thirdsub-step at which a filter installed in the housing is vibrated byvibration of the housing such that crystals of valuable resources,attached to the filter, are separated from the filter.
 3. The resourcerecovery method according to claim 2, wherein after the third sub-stepis performed, the following steps are further performed: analyzingparticle sizes of the crystals of valuable resources filtered out by thefilter using a particle analyzer; and transmitting a control signal foradjusting the frequency of ultrasonic waves generated by the ultrasonicgenerator in accordance with the particle sizes of the crystals ofvaluable resources analyzed by the particle analyzer, to the ultrasonicgenerator.
 4. The resource recovery method according to claim 1, whereinafter the first step, the following steps are further performed:detecting a temperature of the raw water in the first-stage raw watertank, using a temperature detector; and controlling a temperature ofvapor generated by a vapor generator on the basis of the detectedtemperature of the raw water.
 5. The resource recovery method accordingto claim 1, wherein after the sixth step, the following steps areperformed: detecting a conductivity of raw water fed to the crystallizerfrom the n-th-stage raw water tank, using a conductivity meter; andtransmitting an operation signal to the crystallizer by the controllerwhen the detected turbidity and the detected conductivity reachrespective reference values.